1. Field of the Invention
This invention relates to an indirect x-ray image detector suitable for radiology. The term “indirect” means that the x-rays are not directly converted into electrical charges, but rather they are first converted into light using a scintillator, and then light is converted into electrical charges by appropriate means. More particularly, this invention relates to an x-ray detector where the usual array of pixelated photodiodes used to convert light into electrical charges, is replaced by a photoreceptor formed of a thin uniform layer of photosensitive selenium having a multilayer structure.
2. Description of the Prior Art
It is well known to produce indirect x-ray image detectors based on a system comprising a thin film transistor (TFT) matrix deposited on a glass substrate and a corresponding array of photodiodes with an inherent capacitor, as well as an electrode connected to the source of the TFT. The photodiodes are covered with a common transparent electrode and a scintillator. Scanning circuits are connected to the lines of the TFT array while reading circuits are connected to the columns of the TFT matrix.
In such known devices, initially, all the free electrodes of the photodiodes are biased at the same reference voltage. The detector undergoes an exposure. Light is generated in the scintillator which causes the photodiodes to generate and leak charges. The associated capacitance of each photodiode releases a charge proportional to the local exposure resulting in a charge topography over the entire detector. The scanning circuits scan the lines in sequence switching the TFTs and successively connecting the free electrode of each photodiode-capacitance on the active line to the virtual reference voltage of the columns. These columns are connected to a read-out preamplifier which integrates the charge required to bring back the photodiodes to the reference potential. During the time of a line connection, the readout circuit on each line processes the integrated charges and multiplexes them to an output buffer memory where the entire two dimensional image builds up. In these arrangements, the TFTs can also be replaced by switching diodes. Such TFTs or switching diodes and photodiodes are generally made of amorphous silicon. TFTs can also be made of CdSe material. The scintillator is generally made of evaporated CsI.
One such prior art arrangement using a scintillator detector crystal, preferably made of a thallium doped cesium iodide crystal, and an array of photodiodes arranged to receive the scintillator photons, is disclosed in U.S. Pat. No. 5,171,998 of Engdahl et al., for gamma ray imaging.
Another prior art arrangement is disclosed in U.S. Pat. No. 5,198,673 of Rougeot et al. where a scintillator is optically coupled to a large area photosensor which is disposed on a substrate and is electrically coupled to a data read and reset circuit. The large area photosensor is provided with an amorphous selenium photosensitive material that exhibits avalanche multiplication gain when a selected high biasing voltage is applied, but which does not have a multilayer structure and requires a biasing voltage between 100V and 1000V to cause the photosensor to exhibit avalanche multiplication. In this patent, there is also included a protective TFT coupled to the data read and reset circuit to provide overvoltage protection from the high biasing voltage.
A further prior art detector is disclosed in U.S. Pat. No. 4,363,969 of Ong where an array of optically isolated small area scintillators overlay a duophotoconductive sandwich structure that includes a photoconductive layer which acts as a switch Both photoconductive layers can be made of amorphous selenium. This system uses a duo-dielectric sandwich structure, but does not use an active matrix substrate such as a TFT. Also, the construction of an array of scintillators is a complex and difficult task which has not achieved widespread application.
A still further prior art x-ray detector is disclosed in U.S. Pat. No. 5,464,984 of Cox et al. It includes a scintillator, a sensor array of silicon-on-insulator substrate (SOI), processing circuits and a ceramic layer which overlays the processing circuits. No photosensitive selenium is, however, used in this patent.
Also, U.S. Pat. No. 5,585,638 of Hoffman discloses an x-ray detector assembly that comprises an imaging device formed by a scintillator in front of a two-dimensional photodetector array. The active layers are formed on a surface of substrate and a uniform first electrode layer is applied over the entire major surface on one side of the substrate and serves as a high voltage bias electrode. Then, a thin semiconductive layer which constitutes a second electrode, extends over the first electrode layer and provides an electrical characteristic that varies in response to impingement of x-rays. Again, no photosensitive selenium is used therein.
Such prior art devices present a number of disadvantages in terms of high cost and poor resolution. Also, they may be operated only by application of either high voltage or low voltage, thereby restricting their field of use.